Singleton Pattern Creational

Transcription

1 Singleton Pattern Creational Intent» Ensure a class has only one instance» Provide a global point of access Motivation Some classes must only have one instance file system, window manager Applicability» Must have only one instance of a class» Must be accessible from a known location Singleton-1

2 One Singleton Abstract Architecture Eiffel has once function but not static variables More complex architecture + singleton SINGLETON * SINGLETON_ACCESSOR + SINGLE_INSTANCE_CLASS instance CLIENT the_instance the_instance + INSTANCE_ACCESSOR Singleton-2

3 Scenario CLIENT 4 1 INSTANCE_ACCESSOR 3 2 SINGLE_INSTANCE_CLASS Scenario: Get & use instance 1 Create instance_accessor 2 Create the_instance -- only once, thereafter return it 3 Get the_instance 4 Use instance Singleton-3

4 Memory Diagram Singleton-4

5 Participants Singleton Used to type a class as a singleton Single instance class The class that should have only one instance Singleton accessor Declares access point for a single instance Instance accessor Access point for the single instance Client Uses instance accessor to get the single instance Singleton-5

6 One Singleton Class class SINGLETON feature {NONE} frozen the_singleton : SINGLETON -- The unique instance of this class once Result := Current end invariant Enforces single instance property only_one_instance: Current = the_singleton end Singleton-6

7 Singleton Accessor Class deferred class SINGLETON_ACCESSOR feature {NONE} singleton : SINGLETON -- Access to unique instance. -- Must be redefined as once function. deferred end is_real_singleton : BOOLEAN do Result := singleton = singleton end invariant Enforces single instance property singleton_is_real_singleton: is_real_singleton end Singleton-7

8 Instance Accessor Class class INSTANCE_ACCESSOR inherit SINGLETON_ACCESSOR rename singleton as the_instance end feature the_instance: SINGLE_INSTANCE_CLASS -- Create the only instance in the system once create Result.make( ) end end Singleton-8

9 One Singleton Single_Instance Class class SINGLE_INSTANCE inherit SINGLETON end Only need to inherit from SINGLETON class. No other changes Singleton-9

10 One Singleton Consequences Sole instance is extensible by sub-classing Clients use extended instance without modification dynamically Reduce name space Avoids adding global variables storing single instance Singleton-10

11 One Singleton Problem As defined only one SINGLETON is permitted in the system. The once feature in SINGLETON is common to all instances The solution is to have a once feature for each needed singleton The invariant remains in the SINGLETON class Singleton-11

12 Multiple Singletons SINGLETON class as for solution 1» Make the Singleton class deferred» Make the_singleton deferred» Keep the invariant SINGLE_INSTANCE class» Inherit from SINGLETON» Make the_singleton effective Singleton-12

13 Multiple Solution Abstract Architecture * SINGLETON Invariant singleton + SINGLE_INSTANCE_CLASS * SINGLETON_ACCESSOR instance the_instance One per single Instance class CLIENT the_instance + INSTANCE_ACCESSOR Singleton-13

14 Multiple Singleton Class deferred class SINGLETON feature {NONE} the_singleton : SINGLETON -- The unique instance of this class -- Should be redefined as a once function -- returning Current in concrete subclasses deferred end invariant Enforces single instance property only_one_instance: Current = the_singleton end Singleton-14

15 Multiple Singleton Single_Instance Class class SINGLE_INSTANCE inherit SINGLETON feature {NONE} end frozen the_singleton : SINGLETON -- The unique instance of this class once Result := Current end Add to the single instance class Inherit from SINGLETON class. Make the_singleton effective Singleton-15

16 Tradeoffs One singleton technique» Only need to inherit from SINGLETON» Compiler catches invalid create attempts Multiple singleton technique» In addition to inheriting from SINGLETON, need to add the feature the_singleton» Invalid create attempts can only be caught at run time Singleton-16

17 Related Patterns Abstract Factory, Builder and Prototype can use Singleton Singleton-17

18 Singleton Java class AcctNumber Singleton is easy due to having static variables public class AcctNumber { private AcctNumber () { /* Only AcctNumber can construct */ } private static AcctNumber instance = null; Give client access to the single instance public static AcctNumber getinstance() { if ( instance == null ) { instance = new AcctNumber(); } return instance; } } /* See next slide for Singleton data and data access */ Singleton-18

19 Singleton Java class AcctNumber 2 // The singleton data is not directly accessible private int lastacctnumber = 0; // Give clients appropriate access to the data public int getnumber { return lastacctnumber; } public void nextacctnumber { lastacctnumber++; } Singleton-19

20 Singleton Java class AcctNumber 3 Client side // Customer 1 wants a couple of account numbers AcctNumber customer_1 = AcctNumber.getInstance(); customer_1. nextacctnumber(); acct_number = customer_1. getnumber(); use acct_number customer_1. nextacctnumber(); acct_number = customer_1. getnumber(); use acct_number // Customer 2 wants an account number AcctNumber customer_2 = AcctNumber.getInstance(); customer_2. nextacctnumber(); acct_number = customer_2. getnumber(); use acct_number Singleton-20